AM79C982 datasheet(14/26 Pages) AMD

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AMD

PRELIMINARY

1–16

Am79C982

Implementing a 12-Port Unmanaged Hub

Both bIMR4 and bIMR8 chips have an expansion bus

that allows multiple devices to be connected together,

allowing high port count repeaters to be designed. The

operation of the expansion bus is identical for the bIMR4

and bIMR8. Minimum Mode is available in both bIMR4

and

bIMR8

devices.

This

mode

facilitates

the

implementation of the LED display for unmanaged hub.

Figure 2 shows a simple example where one four port

bIMR device and one eight port bIMR device are con-

nected together to form one twelve port logical

unmanaged repeater. As both devices are on the same

board the arbiter function can be local, and the bus

transceivers shown at right in Figure 1 are not

necessary due to the low bus loading in this example. In

this case, the arbiter simply asserts

ACK if one REQ sig-

nal is asserted and

COL if both REQ signals are as-

serted. The arbiter does not assert either signal if neither

REQ is asserted. Note that both

ACK and COL are logic

low when asserted.

The D type flip flop is used to synchronize the reset

signals to both bIMR devices in order to ensure that the

internal 10 MHz clocks of these devices are in phase.

More complex repeaters, including stackable hubs, may

be built using the bIMR family. In these cases, the bus

transceivers may be necessary and the arbitration may

be distributed throughout the system.

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